Fossils (AQA GCSE Biology Combined Science): Revision Notes
Fossils
What are fossils?
Fossils are the remains of living things from millions of years ago. Scientists find them in rocks. They give us evidence for evolution by natural selection.
Fossils help us understand how life on Earth has changed over time. They show us what animals and plants looked like long ago.
Fossils provide crucial evidence that supports Darwin's theory of evolution, showing how species have changed and adapted over millions of years.
How do fossils form?
Fossils can form in three main ways:
1. Traces get preserved
Sometimes we find traces that organisms left behind, such as:
- Footprints in mud that hardens into rock
- Burrows where animals lived
- Root marks from plants
Fossil Trace Example: Dinosaur Footprints
Dinosaur footprints found in Arizona were made 200 million years ago. These trace fossils show us how dinosaurs moved and behaved, even though no body parts were preserved.
2. Body parts get replaced
When an organism dies, hard parts like bones and teeth can be replaced by minerals. This happens as the organism decays slowly.
3. Soft tissues get preserved
Muscles and leaves can sometimes be preserved. This only happens in special conditions where there is:
- No oxygen
- No water
- Low temperatures
These conditions stop the normal decay process.
Why is the fossil record incomplete?
Scientists cannot be completely certain about how life began because the fossil record has gaps. This happens for several reasons:
- Early life forms were soft-bodied - they left few traces that scientists can find
- Geological activity destroyed evidence - earthquakes and volcanic eruptions can destroy fossils
- Dead organisms got eaten - predators and scavengers consumed bodies before fossilisation could begin
- Small, delicate organisms - these are less likely to survive long enough to become fossils
The incomplete fossil record means we must be cautious when drawing conclusions about prehistoric life. Many species may have existed without leaving any fossil evidence behind.
What is extinction?
Extinction happens when there are no living individuals of a species left. Once a species is extinct, it cannot come back.
What causes extinction?
There are several main causes of extinction:
Biological causes
- New diseases kill all individuals
- New predators eat all individuals
- New competitors take all the food or territory
Environmental causes
- Slow environmental changes over millions of years - organisms cannot adapt quickly enough
- Single catastrophic events that happen suddenly:
- Asteroid hitting Earth
- Massive volcanic eruptions
Mass Extinction Event: End of the Dinosaurs
Many scientists think an asteroid impact 66 million years ago caused the extinction of dinosaurs. The impact would have thrown dust and debris into the atmosphere, blocking sunlight and causing dramatic climate change that dinosaurs couldn't survive.
Why do we have more evidence for some organisms?
We have more fossil evidence for animals with bones (vertebrates) than for soft-bodied animals. This is because:
- Bones are hard and preserve better
- Soft bodies decay quickly and rarely become fossils
This preservation bias means our understanding of prehistoric life is skewed towards organisms with hard parts. Many soft-bodied creatures that were probably common in ancient ecosystems are poorly represented in the fossil record.
Key Points to Remember:
- Fossils are remains of organisms from millions of years ago found in rocks
- They form in three ways: traces preserved, parts replaced by minerals, or soft tissues preserved in special conditions
- The fossil record is incomplete because many early organisms were soft-bodied and evidence was destroyed
- Extinction happens when no individuals of a species are left alive
- Extinction can be caused by diseases, predators, competition, environmental changes, or catastrophic events